Question

cos6acosa+sin6asina при а=3π/5

Answer 1

Косинус разности
cos(α-β) = cos(α)·cos(β) + sin(α)·sin(β)

cos(6a - a) = cos (5a) ==>
a = 3π/5

cos(5*3π/5) = cos (3π) = - 1